The present invention relates to securing lids on boxes and more particularly to the use of a specific type of temperature autoregulating heater attached to the lid of the box for heating the lid and thermally activating a sealant disposed between the lid and the vertical on end walls of the box.
The present invention is described as applied to a specific prior art problem but it should be recognized that the teachings relating to attaching lids, sidewalls, bottoms, etc. to a closure are universal in applicability and are described as applied to a particular problem as that problem led to the present invention.
The microwave segment of the electronic industry has grown very rapidly in recent years. The circuits themselve are typically located in small, hermetically sealed boxes. The boxes typically have one or more cavities full of circuitry and components, each of which has a lid. Lids are attached by means of welding, laser welding, soldering, etc.
Lid attachment and removal is a real cost and expense. No one method is preferred and all methods have drawbacks. The very high dollar value of these boxes leads to a need to reenter boxes for rework. Likewise, as many or most of these boxes are for military/aerospace end use, inspection leads to the need to reenter a box. Obviously, the need to machine off a lid to reenter as is often the case now, is a huge drawback to lid welding. Likewise, laser welding is both a high capital cost and high labor cost process.
Soldered lids have their drawbacks. A major drawback has been the flux/contaminant issue. More critical has been the inability to heat the lid contact area uniformly and all at the same time. Soldering irons obviously cannot do the job so that most lids are soldered on in ovens. This method has its own drawbacks, including a relatively long high temperature "soak" of the box and its components. Despite this, the relative ease of re entry makes soldered-on lids desirable. Boxes are typically placed on a hot plate for reopening.
There are a large number of lid shapes and dimensions. There are also many different solders used, hence a wide range of soldering temperatures. Boxes are typically aluminum, plated with tin or gold. Lids are made of aluminum, Kovar, etc., and are also plated.
The present invention utilizes temperature autoregulating heaters which may be any one of the types disclosed in U.S. Pat. No. 4,256,945 to Carter et al, and U.S. patent applications Nos. 586,712, filed Mar. 6, 1984, now U.S. Pat. No. 4,695,715 and Ser. No. 623,238, filed June 21, 1984 which are in the names of Paul Busch and Rodney L. Derbyshire, respectively, a U.S. Patent Application filed concurrently as a CIP application to Ser. No. 623,238 for THIN, FLEXIBLE, AUTOREGULATING STRAP HEATER, the disclosure of which is incorporated herein by reference.
The aforesaid U.S. Pat. No. 4,256,945 discloses an autoregulating heater comprising a layer of magnetic material coated on a conductive layer such as copper. A further or return conductor is disposed adjacent the magnetic layer and insulated therefrom except at an end of each where the two are connected together. The other ends of the return conductor and the magnetic material are connected across a constant current a.c. source, preferably though not necessarily a source in the range of 8 MHz to 20 MHz.2
In operation, below the Curie point of the magnetic metal, the current due to skin effect and the proximity effect of electrical systems, is essentially confined to the magnetic material, i.e. the material adjacent the return conductor. As the temperature of the device approaches the Curie temperature and the permeability of the magnetic material approaches, the current spreads into the copper and the resistance of the heater is reduced. Since current is constant, the Joule heating is greatly reduced, and subsequently the magnetic material falls below its Curie temperature and the cycle repeats. Thus the heater is autoregulating.
Variations of some of the details of the basic design of the aforesaid heater are disclosed in the aforesaid U.S. applications Ser. Nos. 586,712 and 623,238 and a concurrently filed C-I-P application to Ser. No. 623,238, and these designs may also be employed herein although only the embodiment described above is described herein for purposes of brevity.
The concurrently filed application discloses a heater of the above type which is virtually massless and in which the equivalent of the return conductor and the insulation between the return conductor and the magnetic material may be removed.